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Atmosphere (unit)

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Atmosphere
Unit ofPressure
Symbolatm
Conversions
1 atm in ...... is equal to ...
   SI units   101.325 kPa
   US customary units   14.69595 psi
29.92126 inHg
   other metric units   1.013250 bar
760 mmHg
Aneroid barometer for household use from c. 1925

The standard atmosphere (symbol: atm) is a unit of pressure defined as 101325 Pa. It is sometimes used as a reference pressure or standard pressure. It is approximately equal to Earth's average atmospheric pressure at sea level.[1]

History

The standard atmosphere was originally defined as the pressure exerted by a 760 mm column of mercury at 0 °C (32 °F) and standard gravity (gn = 9.80665 m/s2).[2] It was used as a reference condition for physical and chemical properties, and the definition of the centigrade temperature scale set 100 °C as the boiling point of water at this pressure. In 1954, the 10th General Conference on Weights and Measures (CGPM) adopted standard atmosphere for general use and affirmed its definition of being precisely equal to 1013250 dynes per square centimetre (101325 Pa).[3] This defined pressure in a way that is independent of the properties of any particular substance. In addition, the CGPM noted that there had been some misapprehension that the previous definition (from the 9th CGPM) "led some physicists to believe that this definition of the standard atmosphere was valid only for accurate work in thermometry."[3]

In chemistry and in various industries, the reference pressure referred to in standard temperature and pressure was commonly 1 atm (101.325 kPa) prior to 1982, but standards have since diverged; in 1982, the International Union of Pure and Applied Chemistry recommended that for the purposes of specifying the physical properties of substances, standard pressure should be precisely 100 kPa (1 bar).[4]

Pressure units and equivalencies

Pressure units
Pascals Bars Standard atmospheres Pounds per square inch Millimetres of mercury Inches of mercury Technical atmospheres Torrs
1 Pa ≡ 1 N⁄m2 = 1×10−5 bar ≈ 9.86923×10−6 atm ≈ 1.45038×10−4 psi ≈ 7.50062×10−3 mmHg ≈ 2.95300×10−4 inHg ≈ 1.01972×10−5 kgf/cm2 ≈ 7.50062×10−3 Torr
1 bar = 100000 Pa ≡ 100 000 N⁄m2 ≈ 0.98692 atm ≈ 14.5038 psi ≈ 750.062 mmHg ≈ 29.5300 inHg ≈ 1.01972 kgf/cm2 ≈ 750.062 Torr
1 atm = 101325 Pa = 1.01325 bar ≡ 101 325 N⁄m2 ≈ 14.6959 psi ≈ 760.000 mmHg ≈ 29.9213 inHg ≈ 1.03323 kgf/cm2 = 760 Torr
1 psi ≈ 6894.76 Pa ≈ 0.06895 bar ≈ 0.06805 atm ≡ 1 lb⁄in2 ≈ 51.7149 mmHg ≈ 2.03602 inHg ≈ 0.07031 kgf/cm2 ≈ 51.7149 Torr
1 mmHg ≈ 133.322 Pa ≈ 1.33322×10−3 bar ≈ 1.31579×10−3 atm ≈ 0.01934 psi ≡ gn × .001 m × 13595.1 kg⁄m3 ≈ 0.03937 inHg ≈ 1.35951×10−3 kgf/cm2 ≈ 1.00000 Torr
1 inHg ≈ 3386.39 Pa ≈ 0.03386 bar ≈ 0.03342 atm ≈ 0.49115 psi = 25.4 mmHg ≡ gn × .0254 m × 13595.1 kg⁄m3 ≈ 0.0345316 kgf/cm2 ≈ 25.4000 Torr
1 kgf⁄cm2 ≈ 98066.5 Pa ≈ 0.98066 bar ≈ 0.96784 atm ≈ 14.2233 psi ≈ 735.559 mmHg ≈ 28.9590 inHg ≡ 1 kgf⁄cm2 ≈ 735.559 Torr
1 Torr ≈ 133.322 Pa ≈ 1.33322×10−3 bar ≈ 1.31579×10−3 atm ≈ 0.01934 psi ≈ 1.00000 mmHg ≈ 0.03937 inHg ≈ 1.35951×10−3 kgf/cm2 ≡ ⁠101 325/760⁠ = ⁠20 265/172⁠ N⁄m2

A pressure of 1 atm can also be stated as:

≈ 1.033 kgf/cm2
≈ 10.33 m H2O[5]
≈ 760 mmHg[6]
≈ 29.92 inHg[6]
≈ 406.782 in H2O[5]
≈ 2116.22 pounds-force per square foot (lbf/ft2)

The notation ata has been used to indicate an absolute pressure measured in either standard atmospheres (atm)[7] or technical atmospheres (at).[8]

See also

References

  1. "Water Pressures at Ocean Depths". NOAA Pacific Marine Environmental Laboratory. Retrieved 11 October 2022.
  2. Resnick, Robert; Halliday, David (1960). Physics for Students of Science and Engineering Part 1. New York: Wiley. p. 364.
  3. "BIPM - Resolution 4 of the 10th CGPM". www.bipm.org.
  4. IUPAC.org, Gold Book, Standard Pressure
  5. As a unit of measurement, the conventional metre of water (mH2O) is defined as an ideal column of water with density of 1000 kg/m3 under standard gravity gn of 9.80665 m/s2 i.e. 1 m × 1000 kg/m3 × 9.80665 m/s2 = 9806.65 Pa (though in practice the density of pure water is always less). 1 cmH2O = 0.01 mH2O and 1 inH2O = 0.0254 mH2O. BS 350:Part 1:1974 Conversion factors and tables, Part 1. Basis of tables. Conversion factors. British Standards Institution. 1974. p. 49.
  6. As a unit of measurement, the conventional millimetre of mercury (mmHg) is defined as an ideal column of mercury with density of 13595.1 kg/m3 under standard gravity gn of 9.80665 m/s2 i.e. 0.001 m × 13595.1 kg/m3 × 9.80665 m/s2 ≈ 133.322 Pa. 1 inHg = 25.4 mmHg. BS 350:Part 1:1974 Conversion factors and tables, Part 1. Basis of tables. Conversion factors. British Standards Institution. 1974. p. 49.
  7. "The Difference Between An ATM & An ATA". Scuba Diving & Other Fun Activities. March 2, 2008.
  8. BS 350:Part 1:1974 Conversion factors and tables, Part 1. Basis of tables. Conversion factors. British Standards Institution. 1974. p. 50.